The present invention relates to an exhaust emission control system of an engine, and particularly to an exhaust emission control system which is provided in an exhaust passage with an NOx catalyst and an SCR (Selective Catalytic Reduction) catalyst which purify NOx in exhaust gas.
Conventionally, NOx storage catalysts which store (occlude) NOx contained in exhaust gas when an air-fuel ratio of the exhaust gas is lean (i.e., λ>1, larger than a theoretical air-fuel ratio) are known. Such NOx storage catalysts further reduce the stored NOx when the air-fuel ratio is approximately equal to stoichiometric (i.e., λ≈1, approximately equal to the theoretical air-fuel ratio) or is rich (i.e., λ<1, smaller than the theoretical air-fuel ratio). Within a normal operating range of an engine, the engine is operated at the lean air-fuel ratio (λ>1) so as to reduce fuel consumption, although if this lean operation state continues for a while, the amount of stored NOx in the NOx catalyst reaches a limit value and the NOx catalyst can no longer store NOx, which causes NOx to be released. For this reason, the air-fuel ratio is suitably set to be stoichiometric or richer (λ≤1) in order to reduce NOx stored in the NOx catalyst (hereinafter, the control for reducing NOx stored in the NOx catalyst is referred to as “NOx reduction control”). For example, JP2004-360593A discloses an art for enriching an air-fuel ratio of exhaust gas so as to reduce NOx stored in an NOx catalyst when the stored amount of NOx is above a predetermined amount. It will be noted that “λ” is an index of the air-fuel ratio expressed with reference to the theoretical air-fuel ratio, and is a so-called air excess ratio.
Further, an exhaust emission control system has been recently developed to be equipped not only with such a NOx catalyst, but also with an SCR catalyst for selectively reducing and purifying NOx within exhaust gas while using ammonia (NH3) as a reducing agent. Generally, urea water is injected into an exhaust passage upstream of the SCR catalyst and the SCR catalyst purifies NOx by using ammonia generated by urea water. On the other hand, since ammonia is generated when reducing NOx stored in the NOx catalyst, it is also known to purify NOx in the SCR catalyst by using ammonia generated in the NOx catalyst. For example, JP2010-112345A discloses an exhaust emission control system for controlling an SCR catalyst to adsorb ammonia generated in an NOx catalyst during an NOx reduction control, and purifying NOx using the adsorbed ammonia. The exhaust emission control system executes the NOx reduction control only when the adsorbed amount of ammonia in the SCR catalyst is below a predetermined amount, whereas the NOx reduction control is prohibited when the adsorbed amount of ammonia exceeds the predetermined amount, so as to avoid supplying more than an adsorbable amount of ammonia to the SCR catalyst and causing release (slipping out) of ammonia from the SCR catalyst.
However, with the art described in JP2010-112345A, since the NOx reduction control is prohibited whenever the adsorbed amount of ammonia in the SCR catalyst is large, the frequency of executing the NOx reduction control is limited, and thus the NOx purification performance of the NOx catalyst tends to be insufficient. Therefore, it is considered ideal when it is possible to control the release of ammonia from the SCR catalyst resulting from the NOx reduction control, while appropriately ensuring the execution of the NOx reduction control even when the ammonia adsorption amount in the SCR catalyst is large, without prohibiting the NOx reduction control as described above.